If she'd had her fingers on both triggers, she'd likely be bleeding from the back of the front trigger cutting to the bone on her rear trigger finger.

This is what happened on a light 12 bore O/U Baikal on some 2 3/4" magnum 2's - I never did it twice. I'd think the .500 would really do a trick on that rear finger - if not articulated as noted in mausernad9mm's post. If both bingers were in there, the articulation of the front trigger would stop when the index finger hit the front of the guard, thus still cutting the rear trigger's finger.

Looks to me, it simply doubled.